The present invention relates to catalyst elements and catalyst structures for purifying exhaust gases; processes for producing the catalyst elements and catalyst structures; apparatuses for purifying exhaust gases; and processes for purifying exhaust gases by using the apparatus. Particularly, the invention relates to catalyst elements and catalyst structures, used for purifying exhaust gases, in which airflow loss is small, dust in an exhaust gas is hardly deposited, and the contact between the gas, to be treated, with a catalyst is increased to greatly raise the reaction speed; processes for producing the catalyst elements and catalyst structures; exhaust gas purifying apparatuses; processes for purifying exhaust gases by using the apparatus.
Nitrogen oxides (Nox) in exhaust gases discharged from power plants, various plants, automobiles, and others are causative agents for photochemical smog and acid rain. As a method for efficiently removing NOx, an exhaust gas denitration method by selective catalytic reduction using ammonia (NH3) or the like as reducing agent has widely been employed. As catalyst, a titanium dioxide (TiO2) type catalyst containing vanadium (V), molybdenum (Mo), or tungsten (W) as active component has been used. Particularly, a catalyst containing vanadium as one of the active components has become a mainstream of current denitration catalysts since the catalyst is not only high in activity, but also is small in deterioration due to impurities contained in exhaust gases and can be used from a comparatively low temperature (Laid-open Japanese Patent Publication No. Sho 50-128681 and others).
For example, a catalyst structure is known which is produced by using metal laths to which aluminum is flame sprayed, or woven fabrics or nonwoven fabrics of ceramic fibers as substrates, coating a catalyst component thereon, rolling the metal laths or the fabrics to obtain plate-shaped catalysts (catalyst elements), processing them into such a waveform as shown in FIG. 15(a), and then assembling two or more of the waveform catalysts in a stacked state as its cross sectional side view is shown in FIG. 15(b) (Laid-open Japanese Patent Publication Nos. Sho 54-79188 and Sho 59-73053). On the other hand, other catalyst structures are known in which one of other waveforms of catalysts, or one of other waveforms of catalysts and flat catalysts are combined. For example, catalyst structures cross sectional side view of which are shown in FIG. 16(a), (b), or (c) are specifically known (Laid-open Japanese Patent Publication Nos. Sho 53-136656 and Sho 64-12627).
However, catalyst structures produced by stacking conventional catalyst elements have a high pressure loss. Besides, it is difficult to obtain catalyst structures having a sufficiently high strength from conventional catalyst elements since they are apt to deform. Further, conventional catalyst structures have such a problem that a high catalytic activity can not be obtained since contact area between catalyst elements is large leading to a narrow effective catalyst area, and thus dust is apt to deposit in the structures.
First object of the present invention is to provide catalyst elements and catalyst structures having advantages that dust is hardly deposited between catalyst elements and that the elements and structures have a sufficiently high strength even when the thickness of the elements is reduced.
Second object of the present invention is to provide processes by which catalyst elements and catalyst structures can be produced economically and in large quantities.
Third object of the present invention is to provide catalyst structures in which the contact area between catalyst elements is small and thus catalysts are efficiently used.
Fourth object of the present invention is to provide apparatuses and processes for purifying exhaust gases in which the gas flow in a gas flow passage is disturbed to increase the contact between the gas, to be treated, with a catalyst thereby catalytic activity can be improved.
The present invention is summarized as follows:
(1) A catalyst structure for purifying an exhaust gas comprising a stack of two or more catalyst elements and a frame to house the stack,
each of the catalyst elements being formed by bending a rectangular or square plate supporting a catalyst component on its surface into a shape of stairs in the direction parallel to the direction of a pair of sides of the plate at a predetermined interval so that flat plate portions (hereinafter xe2x80x9cflat plate portionsxe2x80x9d means portions corresponding to tread portions in stairs as shown in Figures) and level-changing portions (hereinafter xe2x80x9clevel-changing portionsxe2x80x9d means portions corresponding, for example, to riser (upright) portions in stairs or the portions similar thereto as shown Figures) are alternately formed in the element,
the catalyst elements being stacked such that the position of the level-changing portions are shifted by a predetermined length between adjoining catalyst elements, and
the catalyst elements forming gas flow passages having a rectangular or rhombic cross section between the adjoining catalyst elements.
(2) The catalyst structure for purifying an exhaust gas recited in paragraph (1) above wherein a catalyst element in the stack is contacted with adjoining catalyst elements at three or more points at the ends of the level-changing portions and in the vicinity thereof in total to support each other.
(3) The catalyst structure for purifying an exhaust gas recited in paragraph (1) or (2) above wherein the length of the flat plate portions and the height of the level-changing portions are the same in each of the catalyst elements.
(4) The catalyst structure for purifying an exhaust gas recited in paragraph (1) or (2) above wherein a relation of
p greater than s 
exists between the length p of the flat plate portions and the height s of the level-changing portions in the catalyst elements, and the angle formed between the level-changing portions and the flat plate portions in the catalyst elements is 90xc2x0 or more (as shown in FIG. 2).
(5) The catalyst structure for purifying an exhaust gas recited in paragraph (1) or (2) above wherein the length of each of the catalyst elements is an integral multiple of the sum of the height of the level-changing portions and the length of the flat plate portions.
(6) The catalyst structure for purifying an exhaust gas recited in any one of paragraphs (1) to (5) wherein each of the catalyst elements are formed by applying a catalyst component containing at least two metals selected from the group consisting of titanium, vanadium, molybdenum, and tungsten to a metallic, ceramic, or glass netlike substrate such that the catalyst component is filled in the meshes of the netlike substrate.
(7) A catalyst element used for the catalyst structure for purifying an exhaust gas recited in paragraph (1) above and formed by bending a rectangular or square plate supporting a catalyst component on its surface into a shape of stairs in the direction parallel to the direction of a pair of sides of the plate at a predetermined interval so that flat plate portions and level-changing portions are alternately formed in the element.
(8) A process for producing a catalyst structure for purifying an exhaust gas comprising forming a predetermined length of flat plate portions and a predetermined height of level-changing portions alternately in a belt-shaped substrate for catalyst element so that a stairs-like substrate is prepared, cutting in turn the flat plate portions of the stairs-like substrate thus obtained in the direction parallel to the direction of the edges formed by the level-forming portions and the flat plate portions such that the following relation is established between the whole length W of each of the cut catalyst elements and the distance L between adjacent level-changing portions in each of the elements:
W=nxc3x97L+Lxe2x88x92d 
wherein n represents the number of the level-changing portions per one sheet of the element, and d represents a constant which is smaller than L but larger than 0, to form two or more catalyst elements, and then stacking the catalyst elements (as shown in FIG. 3).
(9) The process for producing a catalyst structure for purifying an exhaust gas recited in paragraph (8) above wherein a catalyst component having a catalytic activity is supported on the belt-shaped substrate for an catalyst element before or after the belt-shaped substrate is cut into catalyst elements having a predetermined whole length of W.
(10) A process for producing a catalyst structure for purifying an exhaust gas comprising cutting in advance a belt-shaped substrate for catalyst element to such a predetermined length that the following relation is established between the whole length W of each of the catalyst elements and the distance L between adjacent level-changing portions in each of the elements to be formed:
W=nxc3x97L+Lxe2x88x92d 
wherein n represents the number of the level-changing portions per one sheet of the element, and d represents a constant which is smaller than L but larger than 0, to obtain two or more unit catalyst elements, forming a predetermined length of flat plate portions and a predetermined height of level-changing portions alternately in each of the catalyst elements so that the position of the flat plate portions and the level-changing portions to be formed are each shifted by a length of d between adjoining catalyst elements, and then stacking the catalyst elements.
(11) The process for producing a catalyst structure for purifying an exhaust gas recited in paragraph (10) above wherein a catalyst component is supported on the belt-shaped substrate for catalyst element before or after the substrate is cut into catalyst elements having a predetermined whole length of W.
(12) A catalyst structure for purifying an exhaust gas comprising a stack of two or more catalyst elements and a frame to house the catalyst elements,
each of the catalyst elements being shaped in stairs-like by alternately forming flat plate portions and level-changing portions therein,
in each of the catalyst elements, the angle formed between the line which connects corresponding vertexes of adjacent level-changing portions in a catalyst element and the flat surface in the level-changing portion being less than 90xc2x0,
the catalyst elements being stacked so that adjoining catalyst elements are contacted with each other at least at a vertex of a catalyst element, and
the catalyst elements forming gas flow passages having a rectangular or rhombic cross section between the adjoining catalyst elements.
(13) The catalyst structure for purifying an exhaust gas recited in paragraph (12) above wherein the catalyst elements are housed with both ends in the direction in which the flat plate portions and level-changing portions are formed in a line being level-changing portions.
(14) The catalyst structure for purifying an exhaust gas recited in paragraph (12) or (13) above wherein the angle formed between the flat surface of the level-changing portion at the ends of the catalyst element and the line which directly connects the vertex of the level-changing portions being 90xc2x0.
(15) The catalyst structure for purifying an exhaust gas recited in any one of paragraphs (12) to (14) above wherein the catalyst elements are formed by applying a catalyst component containing at least two metals selected from the group consisting of titanium, vanadium, molybdenum, and tungsten to a metallic, ceramic, or glass netlike substrate so that the catalyst component is filled in the meshes of the netlike substrate.
(16) The catalyst structure for purifying an exhaust gas recited in any one of paragraphs (12) to (15) above wherein the catalyst elements are stacked through a metallic, ceramic, or glass netlike member interposed therebetween and having a large number of perforated holes.
(17) The catalyst structure for purifying an exhaust gas recited in paragraph (16) above wherein the netlike member interposed between the catalyst elements is a wire cloth, or a woven fabric of ceramic or glass fibers.
(18) The catalyst structure for purifying an exhaust gas recited in paragraph (16) or (17) above wherein the diameter of the wires or fibers in the wire cloth, or woven fabric of ceramic or glass fibers, in the direction perpendicular to the direction of gas flow is larger than the diameter of the wires or fibers in the direction along the direction of the gas flow.
(19) The catalyst structure for purifying an exhaust gas recited in any one of paragraphs (16) to (18) wherein the ceramic or glass fiber woven fabric is reinforced by impregnating it with an inorganic binding material.
(20) The catalyst structure for purifying an exhaust gas recited in paragraph (17) or (18) above wherein a catalyst component containing at least one metal selected from the group consisting of titanium, vanadium, molybdenum, and tungsten is supported on the surface of the wire cloth, or woven fabric of ceramic or glass fibers.
(21) An apparatus for purifying an exhaust gas comprising the catalyst structure defined in any one of paragraphs (1) to (6), and (12) to (20) above placed in an exhaust gas flow passage.
(22) A process for purifying an exhaust gas by using the apparatus for purifying an exhaust gas recited in paragraph (21) above.
(23) A process for purifying an exhaust gas comprising decomposing and removing nitrogen oxides in an exhaust gas by using the apparatus for purifying an exhaust gas defined in paragraph (21) above.